The retinal pigment epithelium (RPE) plays a critical role in the regulation of retinal and choroidal function in normal and disease states. Due to limited availability of human tissues, an in vitro cell culture system is desired. Therefore, we have developed and characterized the primary cell lines of human RPE (HRPE) from donor eyes obtained from eye banks. Using human RPE cell cultures as a model, we conducted investigations to examine the various roles of RPE in the pathophysiology of retinal disorders. Toxoplasma gondii (T. gondii), a protozoan parasite that causes uveitis, Was found to infect and replicate within HRPE cells. Interferons (IFN)-alpha, -beta, -gamma, and tumor necrosis factor-alpha (TNF-alpha) inhibit T. gondii replication in the cells in a dose-dependent manner. IFN-gamma is the most potent and can completely block T. gondii replication. The role of IFN-gamma is confirmed by using monoclonal antibodies (mab) raised against IFN-gamma, which reversed its effects. Antitoxoplasmosis activity of IFNs is by nitric oxide (NO)-independent mechanisms because they do not induce NO production in these cells. HRPE may be a useful model to study the effects of cytokines and drugs on T. gondii growth in retina and responses of HRPE to T. gondii infection. HRPE produce large amounts of NO in the presence of IFN-gamma, interleukin-1 (IL-1), and TNF-alpha. Reverse transcriptase-polymerase chain reaction (RT-PCR ) and Northern blot analysis indicated the expression of an inducible form of NO synthase (iNOS) upon treatment with cytokines. Transforming growth factor-beta (TGF-beta) inhibited cytokine-induced NO production and iNOS mRNA by 60 percent and 75 percent, respectively. Induction of NO by inflammatory cytokines and its regulation by TGF-beta in HRPE suggests a prominent role for RPE in inflammatory and infectious diseases of the retina and choroid. Mechanisms of transport of glutathione (GSH), a peptide involved in the cellular defense against oxidative injury, are studied in HRPE cells. Sodium-independent but temperature-dependent uptake and efflux suggest a carrier-mediated transport of GSH. Presence of low-affinity transporter of GSH was confirmed by Northern blot analysis of HRPE mRNA.